1. Field of the Invention
This invention relates to a method of making a solid electrolytic capacitor, and more particulary pertains to such method including a step of electrodepositing solid electrolyte.
2. Description of the Prior Art
Hitherto known solid electrolytic capacitors have usually been manufactured by the steps of electrolytically oxidizing the surface of a plate or a porous body of a so-called valve action metal, for instance tantalum, titanium, niobium, zirconium, aluminum, hafnium and tungsten, which are capable of being oxidized to form an anode film, thereby producing electrode having a semiconductive dielectric film having a rectifier function. The electrode is impregnated with a material capable of forming a semiconductor, and the impregnated electrode is subjected to pyrolysis to form an oxide semiconductor layer i.e. a solid electrolyte layer contiguous to the anode film and finally the oxide semiconductor layer is coated with a conductive material such as colloidal carbon, silver paint and solder.
In such method it is difficult to deposit the electrolyteforming material such as manganese nitrate Mn(NO.sub.3).sub.2 in a uniform thickness on an anode body. Also, since the deposited manganese nitrate is rapidly decomposed thermally at a high temperature of 200.degree. C. to 400.degree. C. in the pyrolytic process, it is extremely difficult to obtain a dense and uniform layer of pyrolytic manganese dioxide on the dielectric film of the electrode. Therefore, it is necessary to repeat the four steps of immersing the anode body in a solution of manganese nitrate Mn(NO.sub.3).sub.2, pyrolysis, washing with water and re-forming, many times to produce a solid electrolytic capacitor.
Repeating the pyrolytic step many times, however, results in generation of nitrogen oxide gas which is at high temperature and which deteriorates of the dielectric film of the valve action metal both thermally and chemically, and leakage current is increased in proportion to the number of pyrolytic steps involved. Further, with the pyrolytic process alone, the manganese dioxide layer is formed not uniformly but locally, which leads to the short-circuiting between the anode and cathode after the application of the conductive material, so that the capacitor function is lost. This problem is extremely significant in the manufacture of small size solid capacitors and thin film solid capacitors. Furthermore, with a number of repeated pyrolytic steps, the obtained manganese oxide layer has a low density and inferior conductivity and lacks surface smoothness so that the formation or attachment of an electrode is extremely difficult.
In addition to the above-mentioned prior-art method, there has also been developed a method employing electrochemical deposition in forming semiconductive manganese oxide material on the dielectric oxide film. In such method, semiconductive oxide is formed by pyrolysis of a metal salt solution on the dielectric film and then semiconductive oxide e.g. MnO.sub.2 is electrochemically deposited on the thus produced semiconductive oxide film through electrolytic oxidation of an Mn salt solution such as MnSO.sub.4.
However a dielectric oxide film such as Ta.sub.2 O.sub.5 is essentially insulating when the Ta body is positive and not enough current for the electrolytic oxidation can be supplied to the film without inducing electronic breakdown, which means that in this method electrolytic oxidation i.e. deposition has to be performed for longer time with very low current density to get sufficient semiconductive oxide deposition. Further, even in this electrochemical method, a few repeats pyrolytic conversions of Mn(NO.sub.3).sub.2 to MnO.sub.2 is necessary prior to electrodeposition in order to get a smooth and uniform layer of semiconductive oxide via electrolysis. This condition essentially results in thermal deterioration of the oxide film. The thus deposited semiconductive oxide deteriorates the tan .delta. or leakage current of the capacitor, because it is deposited at the defect site of the oxide film. Furthermore semiconductive oxide will be deposited nonuniformly by this method, because oxide defects are distributed nonuniformly on the surface.